Parker says this for "jammy":
"When wines have a great intensity of fruit from excellent ripeness they can be jammy, which is a very concentrated, flavorful wine with superb extract. In great vintages such as 1961, 1978, 1985, 1989, 1990, and 1995, some of the wines are so concentrated that they are said to be jammy."
I, too, tend to find wines commonly described as such... like Aravenel says... not to my liking. More like this definition, "Jammy - One of the wine tasting terms that describes a very ripe wine, often fairly low in acid, so that its fruit flavors and feel resemble jam."
However, I'm sure I've used teh phrase myself to describe a wine I've liked.